A CT clamp is a small current sensor that clips around one of the main cables in your breaker panel or consumer unit. It measures your whole home’s draw and reports it to a smart EV charger, which slows down or speeds up so the house stays within its supply limit. The clamp carries none of the charging current — it only measures.
Key Takeaways
- A CT clamp measures your home’s total load and reports it to a smart charger; it carries no charging current.
- It enables load balancing, solar-surplus charging and multi-charger sharing within a supply limit.
- It is measurement, not protection — breakers and RCD/GFCI devices still do that job.
- Fitting, orientation and the load limit are set by a licensed electrician at installation.
How a CT Clamp Works
CT stands for current transformer. The clamp is a ring that opens, closes around a single conductor — usually a meter tail or the main incoming cable — and senses the magnetic field the current creates. From that field it calculates the current and sends the reading to the charger over a thin signal cable or wireless link. The charger compares the reading with a limit set at installation: if the house is busy, it trims the charging rate; when the house goes quiet, it gives the power back. The clamp never switches charging itself; it informs the charger’s decisions.
Why an Installer Fits One
| Job | What the clamp lets the charger do |
|---|---|
| Protect the main fuse or service limit | Cut charging back when cooking, heating and the car together would overload it |
| Charge from surplus solar | See power being exported and raise charging to absorb it |
| Share one supply between chargers | Keep two chargers’ combined draw inside the wiring’s limit |
| Avoid a bigger service upgrade | Fit a faster charger on a modest supply; the clamp guarantees the limit |
Without a clamp, a charger knows only its own draw — not the oven, the heat pump or a second car. That is why a straightforward install on a generous supply often has no clamp, while a tight supply or solar makes one worthwhile.
What a CT Clamp Is Not
- It is not protection. A clamp cannot trip anything. Overload and fault protection still come from the breaker and the RCD or GFCI, as our guide to whether an EV charger needs GFCI protection explains.
- It is not a meter for billing. It gives the charger a working estimate, not a billing reading — see our explainer on smart meters and EV chargers for the difference.
- It is not optional decoration. Clamped around the wrong cable, or fitted backwards, it makes the charger read nonsense and behave badly: slow at empty times, unchecked at busy ones.
Fitting and Setting Up
The clamp lives inside the consumer unit or panel, on mains cables, so fitting it is a licensed electrician’s job — like the charging circuit itself, which the NEC treats as a continuous load (breaker at 125 percent of charger current). At commissioning the installer sets the house limit, checks the clamp’s direction against a known load, and confirms the charger responds. If you later add solar, a second charger or a bigger supply, tell your installer — the clamp position and limit may both need revisiting. A plug-in portable charger on an existing outlet has no clamp at all — one reason a dedicated circuit matters more for simple setups.
Chargers That Can Use Load Information
Only a charger with load management or solar-matching features can act on a clamp’s readings, so raise it before you buy. Wall chargers such as the EVDANCE EV-ACT5006T 48A Level 2 wall charger (up to 48 A, hard-wired or NEMA 14-50P, app control, ETL/FCC/CE) and the EV-ACT3204PE 22 kW wallbox (three-phase, Type 2, RFID and app) are the kind of units installers pair with load-watching hardware; confirm the arrangement with your installer, because the sensor and its limit are chosen for the house, not the car.
Frequently Asked Questions
Can I fit a CT clamp myself?
No. The clamp sits on main cables inside the panel or consumer unit, which is mains work. A licensed electrician fits it, points it the right way and sets the limit the charger will respect.
Does every EV charger installation need a CT clamp?
No. A simple installation on a supply with spare capacity usually has none. Clamps appear where the supply is tight, solar export is being matched, or several chargers share one feed.
What happens if the clamp fails or is removed?
A well-configured charger falls back to a safe fixed rate or pauses rather than assuming the house is empty. If charging suddenly slows for no obvious reason, a lost clamp signal is one thing your installer will check.
Related Reading
- Do You Need a Smart Meter for an EV Charger?
- Does a Level 1 Charger Need a Dedicated Circuit?
- Does an EV Charger Need GFCI Protection?
- EV Charger Hardwired vs Plug-In: Which Should You Choose?
